Microstructure and Mechanical Properties of AZ31B Magnesium Alloy via Ultrasonic Surface Rolling Process

Ultrasonic surface rolling process (USRP) is a recently developed surface nanocrystallization technology which introduces ultrasonic into surface mechanical rolling technology. By means of USRP, a near-polished,gradient nanostructured surface and high compressive residual stresses were generated on the AZ31B magnesium alloy. The electron backscattered diffraction is used to characterize the microstructure. It was found that the twinning was the main deformation mechanism in tensile test, while grain refinement and twinning were dominant during the USRP. The results show that the thick of affected layer was up to 600 mu m and consisted of three layers: an ultrafine grain layer with thickness of about 180 mu m on the topmost surface, a fine grain transition layer under ultrafine layer up to 420 mu m and a twin layer distributed from the surface to 600 mu m. Combined with Schmid factor (SF), evolution of twins and texture were analysed and were responsible for improvement of strength.